 There is surprisingly little information available about the nutritional value of Atka mackerel given its importance in the diet of Steller sea lions in the western Aleutian Islands.
Scientists at the Marine Mammal Research Unit have been analyzing the energy and fat content of Atka mackerel caught in three regions of the Aleutians to determine whether it differs by size of fish, time of year, or region of Alaska. Initial results are showing surprisingly large (40-50%) changes in fish energy density over the course of the year.
Past feeding studies conducted with captive Steller sea lions at the Vancouver Aquarium have shown that the energy requirements of Steller sea lions also changes dramatically during the year. This has led Dr. David Rosen (UBC) to wonder how seasonal changes in both prey requirements and fish quality might interact. Specifically, whether periods of high demand and low quality food might result in sea lions requiring so much fish that they might have trouble processing all the fish they require to meet their energy needs. This might be particularly troublesome for younger sea lions that have very high energy requirements for their body size.
Some initial modeling of Steller sea lion physiology and Atka mackerel quality yielded some interesting results. In a paper presented at the recent Alaska Marine Science Symposium, Dr. Rosen calculated that a Steller sea lion attempting to wean at 9 months of age would have to consume the equivalent of 15-20% of its own body mass in fish – every day! This amount is very near the demonstrated maximum sustained intake of young Steller sea lions. However, a young sea lion that weaned in the following year would have to consume only about 7-13% of their own mass in fish.
While analysis of fish samples and refinement of the model is ongoing, these initial results provide insight into the decline of Steller sea lions in the western Aleutians by suggesting that reproductive rates could be lower because sea lions are delaying weaning for an additional year.
David Rosen is a Research Associate at the Marine Mammal Research Unit at U.B.C.
Associated Publication:
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Satiation and compensation for short-term changes in food quality and availability in young Steller sea lions (Eumetopias jubatus).
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Rosen, D.A.S. and Trites, A.W. 2004.
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Canadian Journal of Zoology pp. 1061-1069.
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abstract
Foraging theory predicts that animals should proportionately increase their food intake to compensate for reduced energy content and/or prey availability. However, the theoretical intake levels will – at some point – exceed the digestive capacity of the predator. We tested the ability of Steller sea lions (Eumetopias jubatus, Schreber, 1776) to compensate for short-term changes in prey energy density and availability, and quantified the maximum amount of food a young sea lion could consume. Five 1-2 year old captive Steller sea lions were alternately offered herring (high-energy) or capelin (low-energy) each day or every second day. When prey were available on a daily basis the sea lions compensated for differences in the energy content of herring and capelin by consuming sufficient quantities of each (8.3 vs. 14.0 kg d-1, respectively) to maintain an equivalent gross energy intake. When herring was available only on alternate days, the sea lions increased their consumption by 52% to 11.5 kg d-1, which was not sufficient to maintain an average gross intake equal to when herring was available every day. When capelin was available only on alternate days, some animals increased their intake for a few days, but average intake (15.2 kg d-1) was far below levels observed during daily feeding. Generally, the sea lions appeared to reach their digestive limit at a level equivalent to 14-16% of their body mass. Our findings suggest that Steller sea lions can alter their food intake in response to short-term changes in prey quality or availability, but that these variables can quickly combine to necessitate food intake levels that exceed the physiological digestive capacities of young animals.
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